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From: Will Deacon <will.deacon@arm.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <andrea.parri@amarulasolutions.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Kernel development list <linux-kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org, mingo@kernel.org,
	peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com,
	dhowells@redhat.com, Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	akiyks@gmail.com, Palmer Dabbelt <palmer@sifive.com>,
	Daniel Lustig <dlustig@nvidia.com>
Subject: Re: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
Date: Fri, 7 Sep 2018 17:09:50 +0100	[thread overview]
Message-ID: <20180907160950.GH12788@arm.com> (raw)
Message-ID: <20180907160950.Urd7nGaL1HJUPEVOe1LTZ4x9bwWDA1hxuwFDAEwyGiE@z> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1809071145260.1547-100000@iolanthe.rowland.org>

On Fri, Sep 07, 2018 at 12:00:19PM -0400, Alan Stern wrote:
> On Thu, 6 Sep 2018, Andrea Parri wrote:
> 
> > > Have you noticed any part of the generic code that relies on ordinary 
> > > acquire-release (rather than atomic RMW acquire-release) in order to 
> > > implement locking constructs?
> > 
> > There are several places in code where the "lock-acquire" seems to be
> > provided by an atomic_cond_read_acquire/smp_cond_load_acquire: I have
> > mentioned one in qspinlock in this thread; qrwlock and mcs_spinlock
> > provide other examples (grep for the primitives...).
> > 
> > As long as we don't consider these primitive as RMW (which would seem
> > odd...) or as acquire for which "most people expect strong ordering"
> > (see above), these provides other examples for the _gap_ I mentioned.
> 
> Okay, now I understand your objection.  It does appear that on RISC-V,
> if nowhere else, the current implementations of qspinlock, qrwlock,
> etc. will not provide "RCtso" ordering.
> 
> The discussions surrounding this topic have been so lengthy and 
> confusing that I have lost track of any comments Palmer or Daniel may 
> have made concerning this potential problem.
> 
> One possible resolution would be to define smp_cond_load_acquire() 
> specially on RISC-V so that it provided the same ordering guarantees as 
> RMW-acquire.  (Plus adding a comment in the asm-generic/barrier.h 
> pointing out the necessity for the stronger guarantee on all 
> architectures.)
> 
> Another would be to replace the usages of atomic/smp_cond_load_acquire 
> in the locking constructs with a new function that would otherwise be 
> the same but would provide the ordering guarantee we want.
> 
> Do you think either of these would be an adequate fix?

I didn't think RISC-V used qspinlock or qrwlock, so I'm not sure there's
actually anything to fix, is there?

Will

  parent reply	other threads:[~2018-09-07 20:51 UTC|newest]

Thread overview: 112+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-29 21:10 [PATCH RFC memory-model 0/7] Memory-model changes Paul E. McKenney
2018-08-29 21:10 ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-08-30 12:50   ` Andrea Parri
2018-08-30 12:50     ` Andrea Parri
2018-08-30 21:31     ` Alan Stern
2018-08-30 21:31       ` Alan Stern
2018-08-31  9:17       ` Andrea Parri
2018-08-31  9:17         ` Andrea Parri
2018-08-31 14:52         ` Alan Stern
2018-08-31 14:52           ` Alan Stern
2018-08-31 16:06           ` Will Deacon
2018-08-31 16:06             ` Will Deacon
2018-08-31 18:28             ` Andrea Parri
2018-08-31 18:28               ` Andrea Parri
2018-09-03  9:01               ` Andrea Parri
2018-09-03  9:01                 ` Andrea Parri
2018-09-03 17:04                 ` Will Deacon
2018-09-03 17:04                   ` Will Deacon
2018-09-04  8:11                   ` Andrea Parri
2018-09-04  8:11                     ` Andrea Parri
2018-09-04 19:09                     ` Alan Stern
2018-09-04 19:09                       ` Alan Stern
2018-09-05  7:21                       ` Andrea Parri
2018-09-05  7:21                         ` Andrea Parri
2018-09-05 14:33                         ` Akira Yokosawa
2018-09-05 14:33                           ` Akira Yokosawa
2018-09-05 14:53                           ` Paul E. McKenney
2018-09-05 14:53                             ` Paul E. McKenney
2018-09-05 15:00                           ` Andrea Parri
2018-09-05 15:00                             ` Andrea Parri
2018-09-05 15:04                             ` Akira Yokosawa
2018-09-05 15:04                               ` Akira Yokosawa
2018-09-05 15:24                               ` Andrea Parri
2018-09-05 15:24                                 ` Andrea Parri
2018-09-03 17:52                 ` Alan Stern
2018-09-03 17:52                   ` Alan Stern
2018-09-03 18:28                   ` Andrea Parri
2018-09-03 18:28                     ` Andrea Parri
2018-09-06  1:25                 ` Alan Stern
2018-09-06  1:25                   ` Alan Stern
2018-09-06  9:36                   ` Andrea Parri
2018-09-06  9:36                     ` Andrea Parri
2018-09-07 16:00                     ` Alan Stern
2018-09-07 16:00                       ` Alan Stern
2018-09-07 16:09                       ` Will Deacon [this message]
2018-09-07 16:09                         ` Will Deacon
2018-09-07 16:39                         ` Daniel Lustig
2018-09-07 16:39                           ` Daniel Lustig
2018-09-07 17:38                           ` Alan Stern
2018-09-07 17:38                             ` Alan Stern
2018-09-08  0:04                             ` Daniel Lustig
2018-09-08  0:04                               ` Daniel Lustig
2018-09-08  9:58                             ` Andrea Parri
2018-09-08  9:58                               ` Andrea Parri
2018-09-11 19:31                               ` Alan Stern
2018-09-11 19:31                                 ` Alan Stern
2018-09-11 20:03                                 ` Paul E. McKenney
2018-09-11 20:03                                   ` Paul E. McKenney
2018-09-12 14:24                                   ` Alan Stern
2018-09-12 14:24                                     ` Alan Stern
2018-09-13 17:07                                   ` Alan Stern
2018-09-13 17:07                                     ` Alan Stern
2018-09-14 14:37                                     ` Andrea Parri
2018-09-14 14:37                                       ` Andrea Parri
2018-09-14 16:29                                       ` Alan Stern
2018-09-14 16:29                                         ` Alan Stern
2018-09-14 19:44                                         ` Andrea Parri
2018-09-14 19:44                                           ` Andrea Parri
2018-09-14 21:08                                       ` [PATCH v5] " Alan Stern
2018-09-14 21:08                                         ` Alan Stern
2018-09-15  3:56                                         ` Paul E. McKenney
2018-09-15  3:56                                           ` Paul E. McKenney
2018-09-03 17:05               ` [PATCH RFC LKMM 1/7] " Will Deacon
2018-09-03 17:05                 ` Will Deacon
2018-08-31 17:55           ` Andrea Parri
2018-08-31 17:55             ` Andrea Parri
2018-08-29 21:10 ` [PATCH RFC LKMM 2/7] doc: Replace smp_cond_acquire() with smp_cond_load_acquire() Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-09-14 16:59   ` Will Deacon
2018-09-14 16:59     ` Will Deacon
2018-09-14 18:20     ` Paul E. McKenney
2018-09-14 18:20       ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 3/7] EXP tools/memory-model: Add more LKMM limitations Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-08-30  9:17   ` Andrea Parri
2018-08-30  9:17     ` Andrea Parri
2018-08-30 22:18     ` Paul E. McKenney
2018-08-30 22:18       ` Paul E. McKenney
2018-08-31  9:43       ` Andrea Parri
2018-08-31  9:43         ` Andrea Parri
2018-09-06 18:34         ` Paul E. McKenney
2018-09-06 18:34           ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 4/7] tools/memory-model: Fix a README typo Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 5/7] EXP tools/memory-model: Add scripts to check github litmus tests Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 6/7] EXP tools/memory-model: Make scripts take "-j" abbreviation for "--jobs" Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-08-29 21:10 ` [PATCH RFC LKMM 7/7] EXP tools/memory-model: Add .cfg and .cat files for s390 Paul E. McKenney
2018-08-29 21:10   ` Paul E. McKenney
2018-08-31 16:06   ` Will Deacon
2018-08-31 16:06     ` Will Deacon
2018-09-01 17:08     ` Paul E. McKenney
2018-09-01 17:08       ` Paul E. McKenney
2018-09-14 16:36 ` [PATCH RFC memory-model 0/7] Memory-model changes Paul E. McKenney
2018-09-14 16:36   ` Paul E. McKenney
2018-09-14 17:19   ` Alan Stern
2018-09-14 17:19     ` Alan Stern
2018-09-14 18:29     ` Paul E. McKenney
2018-09-14 18:29       ` Paul E. McKenney

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